I would put it down to a few different reasons.
A little over a decade ago was when we started getting huge advances in graphic technology. Systems like the Nintendo 64, Playstation coming out and everyone was getting WOWed over the prettiness factor. Every new game would've had just as many faults as some of the games being released today, but due to the "New Age" everyone welcomed these things. Modern day games are released on 3rd generation consoles. Everyone has seen gorgeous graphics, speed, sound, etc before. It's not just quality, it's expected. If a game comes out with horrible graphics it'll never even get past the advertising stage. All the focus has been diverted from the asthetics of the game and we can now focus on playability.
Another reason is... there are a lot more reviewers these days. I remember back in Generation 1 consoles we had a few magazines and maybe some websites to chose from. All of the sources happened to also make their fortunes of advertising for gaming companies and the like. Gaming magazines were there to tell us the good qualities of the game as well as make money, hard to keep your main financers happy if you're screaming about how bad their game is.
With the focus now almost completely on game play, and the internet introducing thousands more people to pick and pull at the good points and bad points of a game we can easily assume this is why reviewers are becoming more harsh. Also "Nasty" reviews are excellent for shock value, Shock value always sells. Slamming a mainstream game is the metaphorical equivalent of swearing on the today show. Shock is revolution, revolution is excellent and freedom of speech is our most valued freedom.
Keep being harsh people, shock the gaming industry into awesomeness!